A. One-Way Wireless Transmission
There is a movement in the wireless industry towards providing more than simple numeric telephone number messages. These messages are typically originated from personal and office computers and sent to the wireless transmitting system via the telephone network.
These messages are received by the messaging system controller (paging terminal) and processed for transmission via the radio transmitting system. E-mail services have gained tremendous popularity and it is predicted that the current 17 million electronic mail (E-mail) subscribers will grow to 53 million by 1995. The average E-mail message is approximately 450 characters in length and 5 to 8 messages are sent each working day.
Personal computers have become far more compact in size permitting them to "move" with the person verses remaining in a fixed location. It is predicted that within the next few years, the majority of the personal computers will be less than 8 pounds in weight making them extremely convenient as a "portable office". This will make wireless communications a media of choice to accommodate this portable office computer to receive information services and E-mail messages.
This places an extreme burden on the existing radio infrastructure that is allocated for messaging services. Currently, most metro area paging systems operating in the 150 and 450 MHz radio bands are operating at or near full capacity accommodating the current numeric paging subscribers. There is not adequate reserve air time available to accommodate alphanumeric information and E-mail services.
Nine hundred MHz authorizations are currently available for local and regional paging implementation. However, at the current protocol speeds and the projected growth rates, the national channels will undoubtedly reach saturation within the next few years. Currently, one or more of the 900 MHz nationwide paging channels are close to such a saturation. There will be a pressing need to increase the air time efficiency of these radio paging systems.
Furthermore, U.S. Pat. Nos. 4,849,750, 4,851,830, 4,853,688, 4,857,915, 4,866,431, 4,868,562, 4,868,558, 4,868,860, 4,870,410, 4,875,039, 4,876,538, 4,878,051, 4,881,073, 4,928,100, 4,935,732, 4,978,944, 5,012,235, 5,039,984, 5,047,764, 5,045,850, 5,052,049 and 5,077,834 disclose a frequency agile information transmission network and frequency agile data receivers. The above-referenced patents are incorporated herein by reference in their entirety. U.S. patent application Ser. No. 702,939, filed May 20, 1991, entitled "Electronic Mail System with RF Communications to Mobile Processors"; U.S. Ser. No. 702,319, filed May 20, 1991, entitled "Electronic Mail System With RF Communications to Mobile Processors Originating From Outside of the Electronic Mail System"; and U.S. Ser. No. 702,938, filed May 20, 1991, entitled "System for Interconnecting Electronic Mail Systems by RF Communications," disclose a system for linking an electronic mail system to portable computers using one-way wireless transmissions which may use the network and receivers disclosed in the aforementioned patents. These applications are incorporated by reference in their entirety herein.
Collectively, the above improvements utilizing the existing 150 and 450 MHz radio messaging infrastructure produce a significant reduction in the message delivery cost to the wireless subscriber. The cost to deliver a 450 character message with the system described in the above-referenced patents is projected to be approximately 65.cent. versus the $1.50 for a 50 character message the industry is currently offering subscribers. This significant cost reduction will further enhance the growth rate of the wireless information and E-mail service industry.
Adequate reserve radio spectrum is available in the 150 and 450 MHz radio bands in the form of IMTS mobile channels that have been authorized for one- and two-way information transmission. However, a more reliable one-way messaging protocol is needed to accommodate the need for information and E-mail services. An additional requirement for a more air time efficient (faster) message protocol is also needed.
The POCSAG protocol was originally authored by the British Post office code Standardization Advisory Group (POCSAG). It was primarily developed for "tone only" or "semi-synchronous paging format". Unlike a synchronous paging format that must be transmitted continually to maintain synchronization of all the paging receivers, POCSAG is somewhat asynchronous in the respect that it only needs to send a radio signal when messages are about to be delivered. However, a POCSAG transmission is extremely sensitive to atmospheric fades which are discussed below. If a three bit error exists in a transmission of information to a POCSAG receiver, the BCH error correction code embedded therein may be ineffective to prevent the transmission synchronism between the transmitter and receiver from being lost which results in a failure to complete the transmission of the information to the receiver and the receiver reverting into a scanning mode to attempt to lock onto a new transmission containing its identification code. A three bit error is produced by a fade in reception level below the detection level of the receiver for a time interval such as 2 to 4 milliseconds for 1200 and 512 baud data rates respectively.
To gain insight as to the POCSAG protocol, reference is made to FIG. 1 for the following explanation. A POCSAG frame set consists of a PREAMBLE, a SYNC signal, and eight frames that are subdivided into two code words each. POCSAG pagers are synchronous in the respect that once they detect the PREAMBLE and synchronize on the SYNC code word, they only then search for a message in their respective frame. If capcode ID numbers are consecutively assigned, the page is automatically assigned to a respective frame. Taking the binary equivalent of the last three digits of the ID of the pager, it is possible to determine in which one of the eight frames a respective pager would be located.
The POCSAG pager is continually sampling the radio channel, looking for its PREAMBLE. The PREAMBLE is typically 1.125 to 3 seconds in duration, and consists of an alternating string of ones and zeros sent digitally. When the pager samples the radio channel and determines the preamble string, it remains on and searches for the SYNC signal. The SYNC signal is actually a 62.5 millisecond code word that transmits a fictitious address to which the pagers respond. It is an unused address, and therefore does not cause falsing (erroneous turn on) of other pagers. Upon receiving the SYNC code word, the pager searches for a message in its respective frame group.
POCSAG has some inherent inefficiencies by its design. These inefficiencies exist in both 512 and 1200 baud POCSAG pagers, and it is inherent in the architecture of the design of the protocol. In the POCSAG and other digital protocols, the baud rate is also the frequency of the subcarrier (e.g. 512 baud uses a 512 Hz. squarewave subcarrier which is modulated with ones and zeros to encode the parts of a transmission). Referring again to FIG. 1, it should be noted that a frame consists of code word one and code word two. If a POCSAG pager receives a message, the first code word of its frame contains the ID or address information for the pager. It also contains alerting information to indicate to the pager what types of beeps are being issued. Code word two contains the numeric or alphanumeric information. When the pager is in numeric mode, the code word can contain five numeric digits.
However, it should be noted that very few numeric pages are five digits in duration. In fact, in a typical paging system, 98% of the numeric messages are seven digits in duration as illustrated in FIG. 2. Because the numeric message is seven digits in duration, the POCSAG protocol permits a borrowing or an extension into the next frame. The first code word in each frame (which would typically be an address), has a marker bit that indicates whether the code word contains address or numeric information. The remaining two digits of the seven digit example then spill over into the first code word of the next frame. Of the twenty bits of data (five numeric digits) in the next frame word, only eight would be used rendering the balance of the code word useless. POCSAG fills the balance of the code word with "filler code". The second code word of the adjacent frame is also back filled with filler code. The adjacent frame is unavailable for use by any other page. In fact, any message awaiting an adjacent frame two pager, must wait until the next frame group is sent in order to receive the message. The architecture of the POCSAG system requires that a message to a given pager be sent only in its respective frame.
It is obvious that unless great care is utilized in the distribution of receivers to divide the receivers evenly within the frame groups, and that the customer usage in each frame group is equal, severe system air time inefficiencies are obtained. System air time efficiencies can vary between 30 and 60%. A great deal of the air time inefficiency cannot be fully utilized as it is due to the message length (seven digits) and is caused by the insertion of filler codes. If a per message comparison of paging protocols is made, it does not take into account the inherent system inefficiencies when numerous pages are sent. As mentioned previously, POCSAG system efficiencies vary considerably if a great deal of attention is not paid to the proper distribution of ID codes.
To gain some insight as to how the POCSAG paging protocol tends to reduce the air time efficiency, reference is made to FIG. 3. FIG. 3 shows 50 numeric pages that need to be sent via the paging system. For purposes of this example each page is a seven digit numeric page and the pagers are equally distributed between the eight frame groups.
The first problem is that due to each page being seven digits in duration, only an average of 3.5 pages can be sent per frame group. It should also be noted that a seven digit numeric page destined for the eighth frame group necessitates that a SYNC signal be sent. The message then spills over to the first frame of the next frame set. An "overhead" problem that also becomes obvious is that the receivers must be resynchronized after the transmission of the first eight frames. This resynchronization adds to the length of each message sent within the eight frame group. SYNC is 62.5 MS divided by 31/2 pages to apportion overhead. One hundred sixty seven milliseconds of the 267 MS period produces a 62.5% efficiency. Due to the spilling over of messages into their adjacent frames, it is seen that a second problem is precipitated. Assuming that each of the pages arrive in frame group order (e.g. 1234567, 1234567), it is seen that even if the paging terminal can sort to get the maximum 3.5 message per frame group efficiency, that a number of pages destined for the first frame tend to build or stack up. To eliminate this problem, fewer pagers may be issued in the eighth frame group (which spill over into the first frame group). However, the problem is not solved by doing so, and simply a build up of other pages in other frame groups occurs.
The 512 baud POCSAG protocol transmits 2.857 alphanumeric characters per 62.5 MS code word. If an alphanumeric message is transmitted in the first frame, a maximum of 42 characters can be sent before a 62.5 SYNC signal is required.
______________________________________ Maximum Characters Transmitted Frame Before Sync ______________________________________ 1 42.8 2 37.14 3 31.43 4 25.71 5 19.99 6 14.89 7 8.57 8 2.85 ______________________________________
As the national average length of an alphanumeric message is 45 characters, it is reasonable to add the SYNC overhead to the character time. The average E-mail message is considered to be from 150 to 450 characters, which increases the air-time requirements and increases the probability for a reception error.
______________________________________ 21.98 m.s. Per character 2.73 m.s. Frame OVHD per character 24.71 m.s. ______________________________________
Current digital protocols (POCSAG and Golay) are difficult to speed up due to their respective architectures. Attempts to increase POCSAG speed from 512 to 1200 or 2400 baud (subcarrier frequency) have encountered the following problems.
The 1200 and 2400 baud data transmission rates have shortened the data bit time to approximately 800 and 400 microseconds respectively. This short time per bit produces a marked degradation in message receipt reliability.
POCSAG receivers have a BCH error correction scheme that can tolerate only one or two data bits to be erroneous before the transmitted character is unrecoverable. Man-made noise and Rayleigh fading phenomenon become far more prevalent to such a short data bit time. The net result is that the cumulative effect of the error correction scheme that the current digital protocols utilize in combination with the effects of natural and man-made interferences degrade the message receiver's reliability when attempts are made to accommodate information and E-mail services. A three or more bit error represents a fade below the threshold detection level of the receiver which can cause the receiver to loose synchronism with the received information stream and turn off and search for another transmission of its address. A three bit error represents a true message error which results in the loss of at least some data.
Speed per message is actually a relatively poor method to choose which format (type) of pager to utilize on a system. There are subtle differences in the various alpha signaling schemes that have far more impact on the reliability of the paging system and its ability to deliver message information to pagers. Differences in air time efficiencies and the techniques employed to correct erroneously received data by the pager are very important considerations that should be made.
POCSAG (the British Postal protocol) and Golay are digital formats requiring digital transmitters.
The 512 baud POCSAG protocol utilizes 31-bit words, utilizing 11 of the bits for error correction. A 3-bit error in the address, as stated above, causes the message to be missed. This equates to a 4-millisecond fade or noise burst during the address and a 2-millisecond fade error during the message. Twelve hundred baud POCSAG pagers have the same error correction format and air time inefficiencies. The fade resistance is reduced to a 2-millisecond fade during the address and 1-millisecond during the message. Although the number of pagers on a given channel is doubled, the degradation of message reliability due to the-reduced fade resistance becomes noticeable with numeric paging, and markedly poor when long alphanumeric messages are sent.
The Golay protocol utilizes 23-bit words, utilizing 11 of the bits for error correction. The Golay protocol transmits the ID code at 300 baud to increase the decoding reliability. The message is transmitted at 600 baud. The Golay format has an increased reliability in detecting the ID portion of the page due to the slower data rate. However, the overall signalling when the format is analyzed is noticeably slower than 512 baud POCSAG, making it a poor choice to attempt to accommodate alphanumeric information and E-mail services on a radio channel that is currently accommodating tens of thousands of numeric pagers.
A common misconception in the wireless industry concerns the term "baud rate". It is easy to conceive that the higher the baud rate is directly attributable to more pagers per channel. This is in part due to the fact that baud rate as pertaining to computers is thought in "wireless" terms when a calculation as to the character speed is determined. Typically, a computer sends 8 to 11 bits of information per character, and one simply divides that number into the baud rate to determine how fast information is transmitted. The fact is the baud rate is only a portion of the analysis. Unlike wireline computers that are connected with telephone lines, radio transmission requires additional "overhead" to be added to the signaling protocol due to its "one way" nature. Radio paging or one-way information transmission does not have the wire-line or two-way wireless privilege of requesting a second transmitted message if an error is received. Radio paging is a "one time" transmission that is "one way". Manufacturers of radio paging equipment therefore must encode additional information to permit the pager to correct errors caused by radio transmission problems. Instead of 8 to 11 "bits" representing a character, as many as 30 bits may be required. This correction overhead is called "error correction", and in some methods reduces the data transmission rate to the pager by as much as 75%. If half of the 1200 baud data rate is utilized for error correction, the effective data rate is 600 baud. The speed or "baud" rate is further reduced by "overhead" SYNC and "wake-up" preambles that must be sent to prepare the paging receiver prior to the transmission of an actual message.
The operating environment has by far the greatest impact on the reliability of the paging system. Geographical terrain, the operating frequency, the presence of man-made structures, and natural and man-made noise all have a collective impact on the operating efficiency of the current state of art in paging systems. If the radio signal cannot reach the paging receiver, the sensitivity of the receiver or the error correction in the protocol has little purpose. The first requirement of a paging system is to therefore provide a good radio paging signal at all the areas of the paging systems service area.
Geographical terrain of the paging service area determines the number of transmitter sites and the antenna patterns required to provide the necessary "Carey" coverage or service area. The less the variation in terrain, the more evenly distributed the RF field is, and the easier it is to obtain reliable service area coverage. Man-made objects (such as buildings) and geographic variations (hills) tend to cause shadows by blocking the "line of sight" paging signal. In a metropolitan radio environment, the receiver is subjected to a very hostile environment. The paging receiver is subject to multi-path interference, impulse noise, simulcast beats, and in many systems with multiple transmitters and non-synchronization of the transmitters. These phenomena are further compounded by building shadow effects and building penetration attenuation of the signal. All of the mentioned phenomena serve to reduce the reliability of the receiver. Higher power transmitters and multiple transmitters can alleviate a portion of the aforementioned problems, and increase other problems (e.g. multi-path, simulcast beats, and non-synchronized transmitters). It is not a simple problem to resolve, as numerous other problems exist to complicate the reliability of radio messaging services in a given area.
Previously, analog pagers utilized forms of active filters to decode the addressing tones. The active filters in the pagers were very sensitive to any form of phase or any other form of distortion that would modify the sinusoidal signaling wave forms. Analog pagers require a "perfect" sine wave to properly decode and alert the user. Hence the reason for precise phasing of transmitters and synchronized transmitters (simulcast) systems was the necessity to accommodate the active filter decoders in the pagers. Even with synchronization and, proper phasing of the station, the pager would often times decode unreliably when located at the midpoint between two transmitters.
The move to digital encoding methods resulted from these former analog problems. In the early 1980's, digital transmitting and paging products were introduced by manufacturers that did not experience the problems associated with analog pagers. It was thought of as the only method to reliably send numeric data to a paging receiver. Considering that in 1980 analog technology was limited to 300 baud and yet digital technologies could transmit 600 to 1200 baud data, this was correct. It was not an inexpensive move, as literally every piece of equipment in the carriers system required replacement. Paging terminals, base stations, and modems had to be purchased to replace the existing analog equipment. Digital paging also required that additional base stations be added to provide the increased signal strength necessary for reliable data stream reception by the pager. The deficiencies found in the analog technologies were eliminated by the move. Digital pagers did not share the problematic phase errors found in their analog counterparts. Research for new developments in analog technologies were abandoned by the pager manufacturers for several reasons. Analog technology was not as advanced in the early eighties (digital signal processing of analog signals was not available), and by emphasizing sales of digital systems, communications manufacturers could increase sales of replacement base stations and paging terminal dramatically.
In this decade, the advances in analog decoding technology have increased dramatically. Data transmission rates of 19,000 baud on ordinary telephone lines is common (as compared to 300 baud in 1980). Microprocessor assisted digital signal processors are available on a single chip with decoding sensitivities unheard of in 1980.
Even with the increased transmitter maintenance, the cumulative effects of mis-synchronization of the radio transmitters, Rayleigh fading, and man-made noise reduce the reliability of the current digital receivers noticeably. The overall fade tolerance at 2400 baud is less than one millisecond. A gap in the data stream in excess of one millisecond causes the message receiver to terminate the receiving process.
There is a need in the art for a messaging protocol to be compatible on both analog and digital radio transmitting systems. The above-referenced patents disclose a protocol which is compatible with analog and digital transmitters. The protocol disclosed in these patents is approximately 99% reliable for the transmission of a 450 character message but is slower than POCSAG by a factor of approximately four. However, the protocol disclosed in the above-referenced patents like the POCSAG and GOLAY uses only a single serial data stream with error correction bits. This protocol is immune to a fade duration of up to 100 milliseconds. The radiated power required to broadcast this protocol is approximately equal to that required for the POCSAG or Golay protocols.
The majority of the messaging radio transmitting systems have radio channels allocated that utilize 5 KHz transmitter deviation limits and transmitted audio bandwidths that are limited to 300 to 3000 Hz. The digital transmitters currently in service have modems that may limit the data rate to 1200 baud (1200 Hz. subcarrier). Compatibility with the current transmitter infrastructure with any new protocol is imperative to provide universal compatibility. The 1200 baud limitation is typically a constraint by the current design of the integrated modems the digital base stations utilize. The bandwidth of current radio transmitters can accept faster data rates if the bandwidth of the digital modems is increased.
As is apparent from the description of the POCSAG digital protocol above these are fundamental problems of increasing its data throughput. The problems are caused by the propensity of atmospheric serial information transmission to semi-synchronous receivers to be subject to unpredictable interruption caused by atmospheric fades which degrade the atmospheric transmission below the noise threshold of the receiver. As has been pointed out above, a three bit error may cause a total loss of synchronism between an information transmission and a POCSAG receiver from which the receiver cannot recover with the remainder of the transmission after the fade being lost with the receiver going into a search mode to look for another transmission of an address of the receiver. When the probability of a loss of synchronism becomes high, the use of a transmission medium goes down. POCSAG is believed to have a reliability of around 95% for a seven character message which means that a 5% chance exists of losing one or more digits of the transmission. A higher reliability is needed for data transmissions between computers to make one-way serial atmospheric data transmission a widespread methodology.
An analysis of atmospheric transmission using the POCSAG protocol in accordance with accepted mathematical relationships for evaluating atmospheric radio frequency transmissions follows which reveals that the POCSAG protocol is poorly suited to data transmissions of more than a few characters in length.